It is well known in the art to join adjacent metallic work pieces by welding the adjacent work pieces together along a weld seam. For example, pipe lines or pipe systems are fabricated by welding the ends of lengths of pipe together. Typically, the ends of the pipe lengths are beveled, or otherwise prepared (“prepped”), brought into contact or close proximity with each other, and welded together by running a weld bead around the prepped end portions. The finished weld bead generally has a crown portion or profile that extends outward from the peripheral surfaces of the ends of the pipes. It is often desirable, and in some cases necessary, to remove this weld crown so that the welded joint is finished to a smooth outer surface.
Various tools have been employed for the removal of a weld crown from a welded work piece or product. A weld crown is often removed from a weld seam by grinding, typically performed manually using a grinding tool. In processes of pipe fabrication, weld removal may be performed by a specialized tool, such as a portable lathe.
A portable lathe typically employed in pipe fabrication comprises a stationary “clamshell” power unit adapted for fitting about a pipe, the portable lathe having a rotatable cutting ring and a tool holder to attach a cutting tool to the cutting ring. The portable lathe is attached about one of the pipes adjacent to the weld joint, and the cutting tool is rotated about the pipe to remove the weld crown. Generally, the cutting tool is a single point cutting tool that cuts the weld crown in a narrow circumferential path around the pipe, the cutting tool circling about the pipe axis while it is advanced axially to remove the weld crown.
While a portable lathe is effective in removing a weld crown, the portable lathe may create stress risers that run circumferentially around the pipe in the cutting direction of the cutting tool. While the smoothed surface of the weld joint, due to the removal of the weld crown, generally results in a weld joint of greater strength or fatigue life than a weld joint with the weld crown left intact, the circumferential stress risers are considered to limit the strength or fatigue life of the weld joint and limit optimal strength or fatigue life from being reached in such a finished weld joint.
Other techniques have been used, in other environments, for removing a weld bead. For example, Japanese patent application disclosure Hei 6-39431 published Feb. 15, 1994 describes a method of manufacturing spiral steel pipe wherein, during the course of manufacturing the spiral steel pipe, a weld bead crown is cut away using a milling cutter.
It has been discovered that the fatigue life of a welded joint between pipe sections is significantly increased if the weld crown is removed along with a small amount of the parent metal along each side of the weld joint.
In an evaluation of a weld test coupon, using 32 inch diameter, 70-90,000 PSI tensile strength pipe, the grinding of the weld crown to remove the weld crown along with a small amount of the parent metal along each side of the weld joint, followed by polishing the surface of the weld joint, resulted in a three to four times improvement in the fatigue life of the weld joint by comparison to a weld test coupon on which the weld crown was not removed.
Thus, the formation of a contoured, profiled surface along the weld joint (a weld profile) that includes the removal of a small amount of the parent metal along each side of the weld joint, rather than simply removing the weld crown, is desirable to achieve an increase in the quality of the weld joint, and in particular to improve the fatigue life of the weld joint.
However, a manual process of grinding the weld crown, along with subsequent polishing, is tedious, time consuming, and costly. Moreover, such a manual process may lack precision, resulting in non-uniformity of a completed weld profile, characterized by insufficient or excessive removal of material.
An insufficient removal of material may fail to produce the desired weld joint quality, by inadequately extending the weld profile contour into the parent metal alongside the weld joint. An excessive removal of material may result in damage to the work product, by reducing the thickness of the pipe wall below a critical or minimum value. Also, heat generated by the grinder may itself contribute to strength degradation of the work product, based on metallurgical considerations.
A further difficulty in the task of creating a finished weld profile is introduced by any misalignment between adjoining work pieces, including a misalignment caused by dissimilar, ovoid or out-of-round pipe segments.